As is well known, most trumpets of conventional design and construction are implemented with a simple series connection between a coil and a contact breaker within the trumpet itself. The contact breaker is controlled from the coil through a power supply battery. The contact breaker forms, together with the coil connected in series therewith, an electromechanically related system setting the resonance frequency of the trumpet. There exists a growing demand for breakerless trumpets. To fill this demand, it could be assumed of using systems incorporating fixed frequency oscillators, but their application to trumpets entails significant risk and disadvantages, as specified here below:
a factory-applied frequency trimming step is required for each trumpet; PA1 the system may fail to operate as the supply voltage or the operating temperature changes, due to the oscillator frequency spreading from the electromechanical resonance frequency.
As an alternative, the prior art proposes solutions based on the use of power changeover or electronic switches. While being advantageous in several ways, not even this solution is entirely devoid of drawbacks, originating from the large amount of electric power to dissipate through the driver circuit. In fact, the inductive energy will discharge itself through the power switch, and the latter has to be provided with a large-size current sink and a voltage clamping device. This prior scheme requires in any case that a sink element be provided in the form of the power switch coupled to a large-size sink. In addition, with the breaker replaced by electronic power devices, the system performance can no longer be maintained, since a drive signal must be generated and supplied to keep the system fed back. In fact, the contact breaker is also useful to generate the drive signal. The duty cycle adjusting facility is usually provided either in the form of a screw for varying the pressure on the breaker, or of a trimmer of the oscillation frequency.
Examples of such prior schemes are described in U.S. Pat. Nos. 5,293,149; 5,049,853; 5,457,437; 4,871,991; and 5,109,212.
It can be seen, from FIG. 1, which shows the current waveform through the trumpet inductor, that the current increases when the contact breaker is closed. The figure shows this increase as a sinusoidal arc (resonant effect) lasting over T/4 and less than T/2. Upon the contact breaker opening, the current falls gradually according to the inductor own law, down to its zero crossing. At this value, the current remains constant for a time period dependent on the duty-cycle setting by the contact breaker screw.